DHCP with ADSL? I'm guessing you have one of those new fangled combination routers.. generally one would have used a PPPoE client in the past.
As for leases, I don't know where you discovered the
'-r' argument.. but it is indeed invalid, there is no documented way to renew the lease.. subsequent
dhclient(8) attempts generally reassign it.
FreeBSD uses an old version of OpenBSD's
dhclient(8), you could use the ISC DHCP client if you want advanced features:
net/isc-dhcp41-client.
Alternatively if you do have a combo router, there is likely a web interface that would allow you to fiddle with DHCP leases.